Planted in the early 20th century to help control erosion, kudzu is a fast-growing legume that now occupies over 7 million acres in North America, and is expanding at a rate of more than 120,000 acres a year; it is now entering New York and Pennsylvania, and has been found as far north at Maine and Ontario. Like peas and some other crops, the vine" fixes" atmospheric nitrogen, pulling the nutrient straight out of the air instead of having to take it out of the soil. However, kudzu, which can choke out trees and other plants, is so aggressive and fast-growing, it potentially alters the nitrogen cycle in air and soil where it invades. While a graduate student at Stony Brook University, Jonathan Hickman--now a postdoctoral fellow at the Earth Institute--and colleagues investigated its effects in Georgia by comparing nitrogen cycling and nitrogen oxide fluxes in soils invaded by kudzu to those in unaffected soils. "Kudzu is really kind of the poster child for invasive species," said Hickman.

“We expected to see some pretty big impacts on the soils of invaded ecosystems. It turns out that the changes you can’t see in a kudzu invasion are just as dramatic as the ones you can,” Hickman said. Measurements showed that some rates of nitrogen cycling were up to ten times faster in soils where kudzu had invaded. Furthermore, the researchers found that kudzu caused a doubling of emissions of nitrogen oxide from soils--along with volatile organic compounds, the key precursor to ozone pollution in the lower atmosphere, and the main component of urban smog. While ozone in the earth's upper atmosphere protects living things on the surface from harmful ultraviolet rays, in the lower atmosphere, it can damage other plants, and cause respiratory problems for humans.

“We really wanted to see whether an invasive species could affect the atmosphere in a meaningful way,” Hickman said. To do that, Hickman and his colleagues employed a chemical transport computer model to predict the effects of increased nitrogen oxide on air quality. In one scenario, the number of high ozone days in parts of the region increased by seven after an extensive kudzu invasion--an increase of over 35% compared to a scenario without kudzu. Though it was not examined in the study, kudzu also emits isoprene, a volatile organic compound produced in large quantities by certain plant species, which is involved in reactions with nitric oxide to form ozone.

The work establishes a measurable link between the invasive plant and ozone formation, said Hickman--one of the first times a plant has been linked to potentially worse air quality. .

“Air pollution is a risk that hasn’t been considered much in the conversation about invasive species, but it’s something we may have to pay more attention to,” he said.

Because kudzu likes warmer climates and also can take advantage of higher carbon-dioxide levels in the air, its range may continue to grow northward, according to the study.

The other authors are Manuel Lerdau of the University of Virginia, and Loretta Mickley and Shiliang Wu, atmospheric modelers at Harvard University.